Japanese Patent No. 3705960 (Patent Document 1) shows conventional ultrasonic testing. In conventional ultrasonic testing methods for diagnosis of abnormality or abnormal conditions, an ultrasonic wave, which has been entered into an object to be tested and reflected on a discontinuous or discrete interface, namely an abnormal portion, is measured in terms of a pulse (see FIG. 1A).
Japanese Patent Application Publication No. 2004-340807 (JP 2004-340807A) (Patent Document 2) discloses an ultrasonic testing method for detecting minute abnormality such as defects due to insufficient weld by entering an ultrasonic wave into the surface of contact and evaluating waveform distortion of a transmitted wave with respect to the incident wave. In this disclosure, the waveform of the transmitted wave is frequency analyzed with respect to the incident wave to obtain the fundamental and harmonic waves; the ratio of amplitudes between the fundamental wave and the harmonic wave is calculated; and the presence of minute abnormality such as defects due to insufficient weld is determined based on the calculated ratio of amplitudes.
It is difficult by the conventional methods to detect obscure closed cracks as shown in FIG. 1B since the ultrasonic wave transmits such cracks. In contrast therewith, the harmonic and sub-harmonic components as well as fundamental component of the ultrasonic wave are conditionally observed by entering a series of ultrasonic wave pulse signals as shown in FIG. 2 and letting the signals transmit the obscure cracks or closed cracks as shown in FIG. 1B. The series of ultrasonic pulse signals as shown in FIG. 2 are, for example, continuous or successive pulse signals including ½ (half) sub-harmonic components. Phenomena called “acoustic nonlinearity” occur when such continuous-pulse ultrasonic wave signals are used. Some attempts have been made to detect cracks and exfoliations, which could not be found by ordinary methods, by observing such phenomena.